In order to connect to a network, a wireless communication device needs to acquire network synchronization and obtain essential System Information (SI). Synchronization signals are used for adjusting the frequency of the wireless communication device relative to the network, and for finding the proper timing of the received signal from the network. In Third Generation Partnership Project (3GPP) Fifth Generation (5G) New Radio (NR), the synchronization and access procedure may involve several signals:                Primary Synchronization Signal (PSS) allows for network detection in the presence of a high initial frequency error, up to tens of parts per million (ppm). Additionally, the PSS provides a network timing reference. 3GPP has selected m-sequences for PSS.        Secondary Synchronization Signal (SSS) allows for more accurate frequency adjustments and channel estimation while at the same time providing fundamental network information, e.g. cell Identifier (ID). Also, SSS is based on m-sequences.        Physical Broadcast Channel (PBCH) provides a subset of the minimum SI for random access. It will also provide timing information within a cell, e.g. to separate timing between beams transmitted from a cell. The amount of information to fit into the PBCH is highly limited to keep the size down. Furthermore, Demodulation Reference Signals (DMRSs) are interleaved with the PBCH in order for a receiver to receive and demodulate and decode it properly.        
A Synchronization Signal (SS) Block (SSB) for 3GPP NR comprises the above signals (PSS, SSS, PBCH).
In NR, the PBCH transmission scheme is discussed in 3GPP and has the following proposed composition/structure as illustrated in FIG. 1. FIG. 1 illustrates NR-PSS, NR-SSS, and NR-PBCH in a SSB.
In FIG. 1, two Orthogonal Frequency Division Multiplexing (OFDM) symbols are reserved for NR-PBCH transmission. NR-PSS and NR-SSS are defined to be 127 subcarriers wide whereas the NR-PBCH is defined to be 288 subcarriers wide.
A number of (typically rather close in time) SSBs constitute an SS burst. A collection of SS bursts is an SS burst set. The SS burst set is repeated periodically, say, every 20 milliseconds (ms). It has been agreed in 3GPP that, for frequencies below 3 gigahertz (GHz), a maximum of 4 SSBs comprises a burst set, whereas the corresponding maximum is 8 and 64 for 3 to 6 GHz and above 6 GHz, respectively. By allowing multiple SSBs, it is possible to repeat SSB transmissions in different directions by changing the beamforming weights for different SSBs. An SS burst set is transmitted periodically, see FIG. 2 which illustrates a SS burst transmission, and the User Equipment device (UE) can, by using the SSBs in the SS burst set, determine the downlink timing and acquire some fundamental SI from the PBCH. It has been agreed in 3GPP that an NR UE in idle mode can expect an SS burst set transmitted from the network once per 20 ms (in connected mode the UE can be configured to expect SS burst sets once per 5 ms). Hence, once the UE has obtained downlink synchronization, it knows in which slots to expect SSB transmissions.
Looking at FIG. 2, there are SSBs, SS bursts comprising one or more SSBs, and an SSB set comprising one or more SS bursts. Each SSB is drawn in various shaded boxes in FIG. 2. Each different type of shading represents different beam directions in which the SSBs are transmitted. Each SSB with a corresponding beam direction is then repeated with, e.g., 20 ms periodicity. The non-filled boxes represent gaps. A gap in the beginning of a slot may be used for Physical Downlink Control Channel (PDCCH), and a gap at the end of a slot may be used for data or an Ultra-Reliable and Low Latency Communication (URLLC) uplink acknowledgement. The longer gap between SS bursts may be used for other transmissions in either the uplink or downlink.
The SSB (and hence the SS burst and burst set) only uses up to 288 subcarriers (127 for PSS and SSS) while an NR carrier may be significantly wider than that. To allow the bandwidth outside the 288 (or 127) subcarriers used for SSB transmission or symbols in a slot not used for SSB transmission for other data transmission, it is beneficial if the SS burst transmission avoids the first few OFDM symbols in a slot to create a gap to allow for PDCCH transmission.
Although the ambition in 3GPP NR has been to keep broadcast to a minimum, apart from the SSBs, some other channels exist that must be broadcast, i.e. transmitted from the network such that all devices in a cell may receive them, i.e., providing area coverage. 3GPP NR will include Remaining Minimum SI (RMSI) that must be read prior to attempting a random access procedure. Also, paging messages are broadcast in the sense that the network does not explicitly know the whereabouts of the receiver, necessitating a broadcast transmission over a limited geographical area.
RMSI:
RMSI is configured in NR-PDCCH and transmitted in NR Physical Downlink Shared Channel (PDSCH), a.k.a. RMSI.
Paging Signals:
Paging is used to inform UEs in idle or inactive state about the need to connect to the network, or to signal updated system information (SI) or emergency messages. In 3GPP Long Term Evolution (LTE), paging is delivered like any downlink data using the PDCCH and the PDSCH. The paging message, transmitted on the PDSCH, is allocated transmission resources by a scheduling assignment on the PDCCH addressed to the Paging Radio Network Temporary Identifier (P-RNTI) (which is shared by multiple UEs). The delivery channel is cell-specific, since both the reference signals and the scrambling are derived from the Physical Cell Identity (PCI).
It has been agreed in 3GPP that, in NR, the paging message is scheduled by Downlink Control Information (DCI) transmitted on an NR-PDCCH and the paging message is then transmitted on the associated NR-PDSCH. The same principle as in LTE, i.e. delivering paging on a physical channel where the information needed to demodulate the physical channel can be derived from the camping cell PCI, should be supported in NR, too.
The paging furthermore needs to support variable payload, at least since the UE IDs used for paging may be of variable size and paging of multiple UEs during one paging occasion needs to be supported to allow long network Discontinuous Transmission (DTX). It is estimated that the paging payload can be from 10-20 bits for paging a single UE up to several hundred information bits for paging multiple UEs or emergency message transmission, and hence the format needs to support at least such payload variations.
Since paging is typically performed over larger areas, Single-Frequency Network (SFN) transmission involving multiple cells or multiple transmission points serving the same cell is possible, with the benefit of improving the link budget. Hence, it is expected that NR paging will support SFN transmission and allow a common configuration for the Transmission and Reception Points (TRPs) that transmit paging. NR paging will therefore likely support two options for configuration of the information needed in order for the UE to demodulate the paging channel, e.g. DMRS and its scrambling phase: (1) based on camping cell PCI or (2) configurable by the NR base station (gNB) in order to support SFN transmission.
Paging in Beam-Swept Scenarios:
In low frequency bands, omnidirectional or wide beam transmission will typically be used for paging delivery. In higher frequencies, sweeping of narrow beams may be used for paging, similarly to SSB.
Beam sweeping approaches for paging delivery has the disadvantage of large overhead, especially if many UEs need to be paged. The number of repetitions needed to provide coverage at the cell edge depend on the deployment density and the applied modulation and coding. Since paging is typically performed over larger areas, SFN transmission involving multiple cells or multiple transmission points serving the same cell is possible. The SFN mechanism improves the link budget further and should, as mentioned above, also be supported by NR.
3GPP NR is expected to support at least three different delivery mechanisms for paging: (1) omnidirectional or wide beam (e.g., sector) transmission, (2) sweeping of narrow beams, and (3) SFN transmission involving multiple cells or multiple TRPs serving the same cell.